Method and means for compensating amplitude and time drifts in sampled waveform systems

ABSTRACT

Method and means for compensating amplitude and time drifts in sampled waveform systems utilizing a nonsequential scanning pattern of the sampling gate thereby achieving long term averaging for noise reduction. A first sample of the magnitude of the waveform is obtained at a sampling time established with respect to a time base synchronized to the waveform, which sample is taken on a portion of the waveform having zero slope. A second sample thereof is obtained with respect to the time base on a portion of the waveform having a slope of relatively large magnitude with respect to zero slope. The difference between the magnitudes of the waveform obtained during the two sampling times is stored thereby providing a reference by which subsequently occurring time drifts may be compensated. Prior to obtaining each subsequent sample of the waveform, samples are obtained at the two reference sampling times and the difference is obtained between the magnitudes sampled thereat and compared to the stored reference. The error signal provided thereby is utilized to compensate the sampling time of the subsequent sample for time drift. The subsequent sample is compensated for amplitude drift by subtracting the magnitude of the waveform obtained at the zero slope reference sampling time from the magnitude obtained during the subsequent sampling time. The timing drift error signal obtained is also utilized to compensate the two reference sampling times when obtaining subsequent reference samples of the waveform.

United States Patent [72] Inventor Alexander M. Nicolson Concord, Mass.[21] Appl. No. 844,021 [22] Filed July 23, 1969 [45] Patented June 8,1971 [73] Assignee Sperry Rand Corporation [54] METHOD AND MEANS FORCOMPENSATING AMPLITUDE AND TIME DRIFTS IN SAMPLED WAVEFORM SYSTEMS 12Claims, 3 Drawing Figs.

[52] US. Cl 328/155, 307/238, 307/243, 328/135, 328/151 [51] Int. Cl1103b 3/04 [50] Field of Search 307/238, 243;328/135, 151,155

[56] References Cited UNITED STATES PATENTS 3,334,298 8/1967Monrad-Krohn 328/135X 3,364,466 1/1968 Stine i 328/151X 3,368,036 2/1968Carter et al 328/133X 41 PRESET f 40 oisirAL CONVERTER TRIGGER ABSTRACT:Method and means for compensating amplitude and time drifts in sampledwaveform systems utilizing a nonsequential scanning pattern of thesampling gate thereby achieving long term averaging for noise reduction.A first sample ofthe magnitude ofthe waveform is obtained at a samplingtime established with respect to a time base synchronized to thewaveform, which sample is taken on a portion of the waveform having zeroslope. A second sample thereof is obtained with respect to the time baseon a portion of the waveform having a slope of relatively largemagnitude with respect to zero slope. The difference between themagnitudes of the waveform obtained during the two sampling times isstored thereby providing a reference by which subsequently occurringtime drifts may be compensated. Prior to obtaining each subsequentsample of the waveform, samples are obtained at the two referencesampling times and the difference TO ANALOG couwsn q-VWCREMENT 2 t x l36 eEvERATDR I p 5 X i L 4 4L scuecs EXTERNAL 17 21 COMPENSATED ma EREZ37 E SWEE lNPUT wEEp I m m m OUTPUT L L4 DJ 38 3 AMP EXT INT w 3 g 'TIME0 w w svvcH BASE E E E UNIT MAiN i- FRAME I 34 lwNBLANKING 51 WAVEFORMSAMPLING fiflgf SAMPLING I g- INPUT 16x GATE cmcun- I AMPLIFIER l HOLD 1SAMPLING HEAD 18 20 i 52 I LSAMPLE AND 9 SAMPLING OSCILLOSCOPE L HOLD J,

LsAMPLE 15 AND HOLD lNlTlAL VALUE=O POSITIVE SLOPE NEGATIVE SLO E SOURCEOF D.C POTENTIAL METHOD AND MEANS FOR COMPENSATING AMPLITUDE AND TIMEDRIF'IS IN SAMPLED WAVEFORM SYSTEMS The invention herein described wasmade in the course of or under a contract or subcontract thereunder withthe Department of the Air Force.

BACKGROUND OF THE INVENTION 1. Field of the Invention The presentinvention pertains to waveform measuring or displaying systems utilizingsampling techniques.

2. Description of the Prior Art Sampled waveform systems, such assampling oscilloscopy systems, are known wherein a sampling gate isscanned continuously across successive repetitions of a repetitivewaveform and is then returned to an initial position to repeat the scanthereof. Short term random fluctuations with respect to the amplitudeand timing of the waveform, which fluctuations often obscure the shapethereof, may be reduced by taking repeated scans of the waveform therebyaveraging the random fluctuation signals to zero. Long term amplitudeand timing drifts of the system may, however, prevent proper averagingof the random short term fluctuations by imparting a nonzero long termmean to the fluctuation signals. Consequently, measured values of thewaveform may be inaccurate or the shape of the displayed waveform may beindefinitely defined.

The long term amplitude drift may be caused by the high gain widebandamplification circuits required in waveform sampling systems having widebandwidth characteristics. The long term timing drift may ordinarily becaused by drift in the precise time referencing circuits required in asampling system which may display or measure repetitive waveforms havingfractional nanosecond periods.

Long term amplitude and timing drifts of the type described cannotconveniently be reduced by conventional filtering techniques. Forexample, techniques such as chopping are inapplicable because of thebandwidths of the waveforms involved.

Prior art sampled Waveform systems therefore are limited to themeasurement or display of waveforms having nominally high signal tonoise ratios and rise times greater than several hundred picoseconds.Prior to the present invention, for example, the shape of responsecharacteristics of experimental circuits in electrically noisy systemsoften could not be determined at low signal levels utilizingconventional sampled waveform systems of the type described.

SUMMARY OF THE INVENTION The present invention provides a method andmeans to compensate for long term amplitude and time drifts in sampledwaveform systems thereby permitting long term averaging of repetitivewaveforms. Thus, a reduction in random noise fluctuations is achieved.The invention comprises obtaining a first sample of the magnitude of thewaveform at a sampling time established with respect to a time basesynchronized to the waveform, which sample is taken on a portion of thewaveform having zero slope. The magnitude of the sample obtained on thiszero slope portion of the waveform is substantially unaffected by timingdrifts of the system. The magnitude is, however, affected by theamplitude drifts of the system.

A second sample of the waveform is obtained with respect to the timebase on a portion of the waveform having a slope of relatively largemagnitude with respect to zero slope. The sampled magnitude thusobtained is affected by the time drifts of the system and is furthermoreaffected by the amplitude drifts ofthe system to the same degree as themagnitude of the first sample taken. The difference between themagnitudes of the waveform obtained during the two sampling times isstored, thereby providing a reference related solely to the timing ofthe system by which reference subsequently occurring time drifts may becompensated.

Prior to obtaining each subsequent sample of the waveform, samplesthereof are obtained at the two reference sampling times and thedifference is obtained between the magnitudes sampled thereat whichdifference is compared to the stored reference. The error signalprovided thereby is thus related to the time drift of the system and isutilized to compensate the sampling time of the associated subsequentsample therefor. Each subsequent sample is compensated for amplitudedrift by subtracting the magnitude of the waveform obtained at theassociated zero slope reference sampling time from the magnitudeobtained during the sampling time at which the subsequent sample wastaken. The timing drift error signal obtained during a sampling cycle isalso utilized to compensate the two reference sampling times of the nextfollowing sampling cycle.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a graph illustrating atypical waveform to be measured or displayed in accordance with thepresent invention;

FIG. 2 is a schematic diagram, partially in block form, illustrating apreferred embodiment of the invention; and

FIG. 3 is a waveform diagram illustrating signals useful in explainingthe operation of the apparatus of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. I, a waveform10 is illustrated, which is typical of the waveforms which may bemeasured or displayed in accordance with the present invention. Thewaveform 10 is subject to long term amplitude drifts which may cause thewaveform 10 to move upwardly or downwardly with respect to the axesillustrated. The waveform 10 is also subject to timing drifts which maycause the waveform 10 to move rightwardly or leftwardly with respect tothe axes illustrated. In accordance with the method of the presentinvention these amplitude and timing drifts are compensated by samplingthe voltages v, and V of the waveform 10 at the sampling times T, and Trespectively. It is appreciated that the voltage V, will vary as thewaveform I0 moves vertically because of amplitude drift, but will berelatively unaffected as the waveform 10 move horizontally because oftiming drifts. The insensitivity of the voltage V, to timing driftsoccurs as a result of selecting the sampling time T, to occur at aportion of the waveform 10 so that has zero slope.

The sampling time T is selected to occur at a portion of the waveform 10having a relatively large slope. The slope chosen must be of nonzeromagnitude and must be finite. It is appreciated, therefore, that thevoltage V will vary in the same manner as the voltage v, when thewaveform 10 moves vertically because of amplitude drift and willadditionally be affected as the waveform 10 moves horizontally becauseof timing drift.

At an initial time established with respect to the sampling procedure ofthe waveform 10, the voltages V, and V are measured and the differencetherebetween is stored. This initial difference between the voltages Vand V may be designated as (V,V,) Since the voltage V varies as afunction of both the amplitude and time drifts and the voltage V variesas a function of the amplitude drift alone, the quantity (V V,) isrepresentative solely of the timing drift of the system. The difference,therefore, between the quantity (V V,), measured at a subsequent timewith respect to the initail time, and the stored quantity (V -V,),,, isrepresentative of the timing drift that has occurred between the initialtime and the time the measurement was taken. A quantity related to thistiming drift error may be combined with the sampling times T, and T tothereby shift T and T back to the points on the waveform 10 which wereoriginally sampled at the initial time, thereby compensating thesampling system for timing drift.

The points of the waveform V are successively sampled at sampling timeT;,, which sampling time is incrementally advanced from left to rightacross the waveform as indicated by the arrow 11. Prior to obtainingeach of the V samples, a measurement of the quantity (V,V,) is obtainedand compared to the stored quantity (V V,),, and the error signalthereby obtained is utilized to compensate the position of the samplingtime T This timing drift error signal is also utilized to compensate thesampling times T and T of the next obtained timing reference samples.Thus, it may be appreciated that the timing of the sampling system iscontinuously adjusted to follow the rightwardly and leftwardly timingdrifts of the waveform 10.

In order to compensate the samples V for upward and downward amplitudedrifts of the waveform 10, the voltage sample V associated with a Vsample is subtracted therefrom providing a quantity designated as (V -VSince the voltage samples V and V vary to the same extent with respectto each other due to amplitude drifts of the waveform 10, the quantity(V -V represents a sample thereof compensated for amplitude drifts ofthe system.

Referring now to FIG. 2, a preferred embodiment of the apparatusutilized in practicing the invention is illustrated wherein a waveformto be measured or displayed is applied generally as an input to aconventional sampling oscilloscope 15. The sampling oscilloscopeincludes a sampling gate 16 and a sample and hold circuit 18 whichtogether comprises an oscilloscope sampling head 19. The samplingoscilloscope 15 further includes a time base unit 17, a samplingamplifier and an oscilloscope main frame unit 21. The input waveform isapplied to the sampling gate 16 which is enabled by a signal provided bythe time base unit 17 in a manner to be explained. The output from thesampling gate 16 is applied as an input to the sample and hold circuit18. The sample and hold circuit 18 is triggered by a signal provided bythe time base unit 17 in a manner to be described. The sample and holdcircuit 18 provides signals representative of the input waveform to thesampling amplifier 20. The sampling amplifier 20 provides the signalsfor the vertical deflection circuits of the oscilloscope main frame 21.The horizontal sweep signal, applied to the horizontal deflectioncircuits of the oscilloscope main frame 21, is provided by the time baseunit 17 in a manner to be explained. The time base unit 17 additionallyprovides an unblanking signal to the intensifying circuits associatedwith the cathode-ray tube 42 of the oscilloscope main frame 21. The timebase unit 17 generates a sampling waveform, illustrated by the timingwaveform A of FIG. 3, which is synchronized to the input waveform viathe synch input to the unit 17. The sampling waveform A provides a timebase with respect to the input waveform 10 and is utilized in the timebase unit 17 to provide triggering signals to the sampling gate 16 andthe sample and hold circuit 18 in a manner to be explained. The sweepsignal provided by the time base unit 17 to the oscilloscope main frame21 is either generated internally thereby or is provided via an externalsweep input thereto and is selectively controlled by anexternal-internal switch 22.

The sampling amplifier 20 of the oscilloscope 15 provides signalsrepresentative of the input waveform simultaneously to sample and holdcircuits 51, 52 and 53. The sample and hold circuits 5], 52 and 53 aretriggered by a trigger generator 24 via a three-position rotary switch25 in a manner to be explained. It is to be appreciated that electronicswitching circuits functionally equivalent to the rotary switch 25 maybe substituted therefor. The output signals from the sample and holdcircuits 51 and 52 are applied respectively to the inverting andnoninverting inputs of an operational amplifier 26. The output signalfrom the amplifier 26, which signal is representative of the differencebetween the signals applied to the inputs thereof, is applied to thenoninverting input of an operational amplifier 27. A source of DCpotential 28 provides a variable DC voltage, via a potentiometer 29, toan inverting input of the amplifier 27. The output signal from theamplifier 27, which signal is representative of the difference betweenthe signal from the amplifier 26 and the signal from the potentiometer29 is, applied as an input to a multiplier 30. The multiplier 30multiplies the signal from the amplifier 27 by a predeterminedcoefficient B. The output signal from the multiplier 30 is applied asthe input to an integrator 31. The integrator 31 provides a feedbacksignal which is representative of the integrated error signal providedby the amplifier 27. The feedback signal from the integrator isselectively applied via a switch 32 to the inverting and noninvertinginputs of an operational amplifier 33. The amplifier 33 and the switch32 determine the polarity of the feedback signal in accordance with theslope of the input waveform 10 illustrated in FIG. 1 at the point Vthereof. The output of the amplifier 33 provides an input signal to anoperational amplifier 34. The output of the amplifier 34, in turn,applied to the external sweep input of the time base unit 17 of thesampling oscilloscope 15 as previously explained.

The sample and hold circuits 51 and 53 provide signals to the invertingand noninverting inputs of an operational amplifier 35 respectively. Theoutput of the amplifier 35 provides compensated output samples of theinput waveform in a manner to be explained.

The circuit consisting of the subassemblies of the sampling oscilloscope15, the sample and hold circuits 51 and 52, the amplifiers 26 and 27,the multiplier 30, the integrator 31, the switch 32 and the amplifiers33 and 34, comprises a feedback loop for maintaining the sampling timesT and T at predetermined points on the waveform 10 during a samplingprocedure thereof in a manner to be explained.

Circuits are included in the apparatus of FIG. 2 for providing signalsrelated to the control of the sampling times T T and T A source of DCpotential 36 provides variable sampling time positioning signals I, andt as illustrated in FIG. 3, via potentiometers 37 and 38 respectively toelectrical contacts 1 and 2 respectively of a three-position rotaryswitch 39. It is to be appreciated that electronic switching circuitsfunctionally equivalent to the rotary switch 39 may be substitutedtherefor. The rotary switch 39 is coupled to the rotary switch 25, aswell as to the trigger generator 24, to provide synchronous operation ofthe disclosed apparatus in a manner and for reasons to be explained. Thetrigger generator 24, as previously described, provides triggeringsignals to the sample and hold circuits 51,52 and 53 and in additionprovides incrementing signals to a counter 40 via electrical contact 2.The counter 40 may, for example, be a conventional binary pulse counter,the digital output of which provides the input signals to adigital-to-analog converter 41. The digital-to-analog converter 41provides an analog signal which is illustrated in FIG. 3 to electricalcontact 3 of the switch 39 in a manner and for reasons to be explained.The instantaneous value of the analog signal t is representative of theinstantaneous value of the digital count signal provided by the counterd0.

The wiper of the switch 39 sequentially couples the signals t,, 1 andt;,, thereby providing the external sweep input to the samplingoscilloscope 15 as previously described.

The apparatus illustrated in FIG. 2 is initially conditioned foroperation by adjusting the potentiometers 29, 37, and 38; thecoefficient B of the multiplier 30; the switch 32; and the counter 40 inaccordance with the following procedure.

The sampling times T and T are first positioned with respect to the timebase synchronized to the waveform 10 by adjusting the potentiometers 37and 38, respectively. The potentiometers 37 and 38 may be adjusted bysetting the external-internal switch 22 to the internal position. Withthe switch 22 in the internal position, the oscilloscope 15 will displaythe input waveform 10 on the cathode-ray tube 42 in the conventionaloperating manner of a sampling oscilloscope. Although the displayedwaveform 10 may be contaminated by noise signals, the general shapethereof may be observed. Particularly, a portion of the waveform 10having zero slope and a portion thereof having a slope of relativelylarge magnitude with respect to zero slope may be determined. With theswitch 39 rotating in the direction of the arrow and with theexternalinternal switch 22 now set to the external position, threepoints of the waveform 10, indicated by V V and V;, on FIG. 1 will bedisplayed on the cathode-ray tube 42 in a manner to be explained. Thethree points V,, V and V correspond to the sampling times T,, T and Trespectively. The sampling times T,, T and T are determined by the threevoltages 1,, t, and respectively, in a manner to be discussed. Thevoltages 1,, t and are provided by the potentiometers 37 and 38 and thedigital-to-analog converter 41, respectively, as previously described.

It may now be observed that as the potentiometers 37 and 38 areadjusted, the respective sampling times T, and T change correspondinglywith respect to the time base and the respective points V, and V movecorrespondingly along the waveform 10. Thus, by adjusting thepotentiometer 37 so that the point V, is centered approximately on thepreviously observed portion of the waveform having zero slope and byadjusting the potentiometer 38 so that the point V is centeredapproximately on the previously observed portion of the waveform 10having a relatively large slope, the sampling times T, and T may beinitially positioned with respect to the system time base in order tosample the two required reference points of waveform 10 as previouslydiscussed.

The proper initial positioning of the sampling times T, and T may beverified by alternating the position of the externalinternal switch 22between external and internal, thereby alternately displaying thewavefonn 10 and the three points V,, V, and V:, on the cathode-ray tube42. It may thus be observed if the point V, is properly positioned onthe portion of the waveform 10 having zero slope and if the point V isproperly positioned on the portion of the waveform 10 having arelatively large slope and appropriate adjustments made to thepotentiometers 37 and 38 if they are not. Alternatively, the threepoints V,, V, and V may be examined simultaneously with the waveform 10by alternating the position of the external-internal switch 22 at asufficiently rapid rate so that the points and the waveform appear to bedisplayed simultaneously on the screen of the cathode-ray tube 42. Theswitch 22, therefore, may be comprised ofelectronic components.

The potentiometer 29 may now be adjusted thereby to store an initialreference value (V V,) of the quantity (V V,). In order so to set thepotentiometer 29, the feedback loop previously described is opencircuited, for example, by setting the coefficient B of the multiplier30 to zero. With the external-internal switch 22 positioned to externaland with the rotary switches 25 and 39 rotating in the direction of thearrows, the potentiometer 29 is adjusted until the voltage at the pointX becomes zero. Since the output of the amplifier 26 is representativeof the quantity (V -V,), and the output of the amplifier 27 isrepresentative of the difference between its input signals, when theoutput of the amplifier 27 is zero, the voltage provided by thepotentiometer 29 will be equal in magnitude to the voltage provided bythe amplifier 26. Therefore, the volt; age provided by the potentiometer29 is representative of the quantity (V,-V,) at the initial time atwhich the adjustment thereto is made, which quantity is designated as(V,-V,),,.

The switch 32 may now be adjusted in accordance with the slope of thatportion of the waveform 10 on which the point V had been positioned. Ifthe slope thereof is positive, the switch 32 is set to the positiveslope position thereby providing a feedback signal from the amplifier 33whose polarity is not inverted. If, however, the slope of the portion ofthe waveform 10 on which V, had been positioned has a negative slope,the switch 32 is set to the negative slope position thereby providing afeedback signal from the amplifier 33 whose polarity is inverted.

The coefficient B of the multiplier 30 may now be adjusted to itsinitail condition. As previously described, the circuit in which themultiplier 30 is included comprises a feedback loop. It is understoodthat the coefficient B must be adjusted so that the feedback loop gaindoes not exceed unity thereby to preclude oscillation of the system.Additionally, the feedback loop gain should be as large as possible,although not exceeding unity, so that the loop may provide as rapid aresponse as possible. ldeally, the coefficient B should be adjusted sothat the feedback loop gain is precisely unity. With theexternalinternal switch 22 positioned to external and the switches 25and 39 rotating in the direction of the arrows, the coefficient B of themultiplier 30 may be set to its initail condition. ln order so to setthe coefficient B, a square wave potential may be superimposed acrossthe potentiometer 29 and the resultant waveform at the point Y observedon an auxiliary oscilloscope, not shown. The coefficient B may beadjusted so that the observed waveform has as fast a rise time aspossible without overshoot. Under this condition, the coefficient B ofthe multiplier 30 will be properly adjusted.

The counter 40 may now be preset to an initail value in accordance withthe initail point of the waveform 10 to be sampled. With theexternal-internal switch 22 set to the external position and the switch39 set to position 3, the counter 40 may be preset, in any convenientmanner, to a number so that the digital-to-analog converter 41 providesa voltage I, to properly position the initial point of the waveform 10to be sampled.

in order to understand the operation of the apparatus illustrated inFIG. 2, it will first be necessary to describe the operation of thecircuits of the sampling oscilloscope 15. The time base unit 17generates a sawtooth waveform of the shape illustrated by waveform A ofFlG. 3. The cycles of the waveform A are synchronized in a conventionalmanner to the cycles of the repetitive input waveform 10 by means of thesynch" input to the time base unit 17. The waveform A therefore providesa time base for the system with respect to the input waveform 10. Withthe external-internal switch 22 set to the internal position, the timebase unit 17 generates a slowly increasing ramp voltage, notillustrated. The slowly increasing ramp voltage is combined in the timebase unit 17 with the waveform A so that a trigger pulse is provided tothe sampling head 19 whenever the waveform A intersects the slowlyincreasing ramp. The trigger pulses thus provided are utilized totrigger the sampling gate 16 and the sample and hold circuit 18. Thus,at each occurrence of a trigger pulse, the sampling gate 16 transmits asample of the input waveform to the sample and hold circuit 18 whereinthe sample is held until the next occurring sample is obtained. Sincethe waveform A, which is synchronized to the input waveform coacts withthe slowly increasing ramp voltage as explained, the samples obtained bythe sampling gate 16 progress slowly across a cycle of the repetitiveinput waveform.

The samples held in the sample and hold circuit 18 are provided via thesampling amplifier 20 to the vertical deflection circuits of the mainframe 21. The slowly increasing ramp voltage, generated by the time baseunit 17, provides the horizontal sweep deflection signals to the mainframe 21. The trigger pulses provided by the time base unit 17 are alsoapplied to the grid electrode of the cathode-ray tube 42 thereby toprovide unblanking signals for the electron beam. Thus, successivelyoccurring samples obtained across a cycle of the input waveform 10 isdisplayed on the cathode-ray tube 42 thereby manifesting the shapethereof.

The sampling amplifier 20 may include an integration function forsmoothing the sampled signals provided thereto by the sample and holdcircuit 18, thus, providing reduction in random noise fluctuations ofthe input waveform 10.

For purposes of illustration, the operation of the apparatus disclosedin FIG. 2 will now be described with respect to the input waveform 10 ofFIG. 1, After the apparatus has been initially conditioned for operationby adjusting the components thereof as previously described, the devicemay provide an output signal via the amplifier 35 which signal isrepresentative of the sampled input waveform 10 compensated for theamplitude and time drifts of the system. With the external-internalswitch 22 positioned to external, the voltages 1,, t and 1 asillustrated in FIG. 3 are supplied by the amplifier 34 and provided tothe time base unit 17 to replace the internally generated slowlyincreasing horizontal sweep ramp signal previously described. With therotary switches 25 and 39 rotating in synchronism in the direction ofthe arrows, the waveform 10 is sampled in the direction of the arrow 11beginning at the amplitude axis of FIG. 1.

When the switches 25 and 39 are positioned at their respective contacts1, the voltage I, is applied via the amplifier 34 to the time base unit17. The time base waveform A intersects the voltage I, as illustrated inFIG. 3, providing the trigger pulses to the sampling head 19corresponding to the sampling time T in the manner previously described.Consequently, the sampling amplifier provides the voltage sampled atsampling time T to the sample and hold circuits 51, 52 and 53. A triggerpulse from the trigger pulse generator 24 enables the sample and holdcircuit 51 via the contact 1 of the switch thereby to store the sampleobtained. The trigger generator 24 is coupled to the rotary switches 25and 39, as indicated by the dashed lines, to provide a trigger pulse tothe electrical contacts l, 2 or 3, respectively, of the switch 25 justprior to the time that the wiper of the switch 39 breaks electricalcontact with the respective contacts 1, 2 or 3 of the switch 39.

The time interval during which the wiper of the switch 39 connects tothe contact 1 thereof determines the number of cycles of the time basewaveform A that intersect the voltage t,. The time interval that thewiper of the switch 39 connects to the contact 1 is dependent upon therate of rotation of the switch 39. Therefor, the rate of rotation of thewiper of the switch 39 determines the number of samples obtained by thesampling gate 16 at the sampling time T,, thus determining the extent ofthe integration performed by the sampling amplifier 20 on the noisesignals distorting the waveform 10 at the sampling time T With therotary switches 25 and 39 now positioned to their respective contacts 2,the voltage is applied to the time base unit 17 via the amplifier 34which voltage is illustrated in FIG. 3. In the manner described withrespect to position 1 of the rotary switches 25 and 39, the samplinggate 16 provides samples of the waveform 10 obtained at sampling time Tto the sampling amplifier 20 via the sample and hold circuit 18. Thesampled voltage provided by the sampling amplifier 20 is entered intothe sample and hold circuit 52 by means of a trigger pulse from thetrigger generator 24 via the contact 2 of the switch 25. This pulse fromthe trigger generator 24 is also utilized to increment the counter 40 byone count for a reason to be explained.

It is appreciated that the longer a point of the waveform 10 is sampledat a particular sampling time, the greater will be the signalenhancement due to the integration function of the sampling amplifier20. Therefore, the longer the switch 39 dwells at the positions thereof,the better will be the noise cancellation in the system. It is to beunderstood, however, that the rate of rotation of the switch 39 must bechosen sufficiently rapid so that the system experiences negligibleamplitude and time drifts during one rotation thereof. It may also beappreciated that for further signal enhancement, low-pass filteringcircuits may be interposed between the sampling am plifier 20 and thesample and hold circuits 51, 52 and 53.

The voltages sampled at the sampling times T and T and held respectivelyin the sample and hold circuits 51 and 52 are applied to the amplifier26 wherein the difference therebetween, (V,V,), is obtained. Theamplifier 27, in turn, provides the difference between the quantity VV,) and the previously stored quantity (V -V,) provided by thepotentiometer 29. The error signal provided by the amplifier 27 ismultiplied in the multiplier 30 by the coefficient B and thereafterapplied to the integrator 31. The integrator 31 provides a feedbacksignal to the amplifier 34, the polarity of which signal is adjusted bythe switch 32 and the amplifier 33 in accordance with the slope of thewaveform 10 at sampling time T as previously described. If the timing ofthe system has not changed since the time at which the potentiometer 29was initially adjusted, the feedback signal applied to the amplifier 34will be 'of zero magnitude. If, however, the timing of the system hasdrifted since the initial adjustment of the poten tiometer 29, themagnitude of the feedback signal will be representative of the magnitudeof the timing drift and the polarity of the feedback signal will berepresentative of the direction thereof.

With the rotary switches 25 and 39 now positioned to their respectivecontacts 3, the voltage which is determinant of the sampling time T isapplied to the amplifier 34. The amplifier 34 combines the feedbacksignal provided by the integrator 31 with the voltage t therebycompensating the sampling time T for the timing drift of the system. Thevoltage sampled by the sampling circuits of the sampling oscilloscope 15at the sampling time T is entered into the sample and hold circuit 53 bymeans of a trigger pulse from the trigger generator 24 via position 3 ofthe switch 25. Since any amplitude drift suffered by the system hasaffected both the voltages V and V held respectively in the sample andhold circuits 5] and 53 to the same degree, the quantity (V -VJ providedby the amplifier 35 is representative of a sample of the waveform 10obtained at a sampling time T with respect to the time base of thesystem and compensated for both amplitude and timing drifts thereof.

As the rotary switches 25 and 39 continue to rotate in the direction ofthe arrows, repeatedly resampling the waveform 10 at the referencesampling times T and T the feedback signal provided by the integrator 31to the amplifier 34 continually adjusts the sampling times T, and Tthereby maintaining the timing of the system constant with respect tothe waveform 10 in accordance with the initial timing condition, (V -V,)as initially established. Furthermore, as the rotary switches 25 and 39continue to rotate in the direction of the arrows, the error signalprovided by the amplifier 27 continuously adjusts the feedback signalvia the integrator 31 thereby to continuously compensate the system fortiming drift thereof thus maintaining the timing of the system time baseconstant with respect to the waveform 10.

As the rotary switch 25 continuously rotates in the direction of thearrow, the pulses from the trigger generator 24 continuously incrementthe count in the counter via the contact 2 of the switch 25. As thecount in the counter incrementally increases, the digital-to-analogconverter 41 coupled thereto, provides the staircase-shaped voltage 1;,illustrated in FIG. 3. Therefore, in the manner previously described,the sampling time T is incrementally positioned across the waveform 10in the direction of the arrow 11 as indicated in FIG. 1. The counter 40may be internally connected in a conventional manner to reset back toits initially preset condition after attaining a predetermined count.The predetermined count may be selected so to cause the sampling time Tto reach a predetermined end point of the cycle of the waveform 10 whenthe counter 40 is preset back to its initial condition.

It is therefore appreciated from the foregoing description that eachsample of the waveform 10 obtained at the sampling time T is preceded bytwo samplings of the waveform 10 at the reference sampling times T, andT Thus, the positioning of the sampling times T T and T; with respect tothe waveform 10 are continuously compensated for the timing drifts ofthe system thereby effectively locking the time base of the system tothe waveform 10. It is furthermore appreciated that the samples of thewaveform 10 obtained at the sampling times T are compensated foramplitude drift of the system by means of the subtraction performed bythe amplifier 35.

lt will therefore be appreciated that a slow scan of the repetitiveinput waveform 10 may be obtained with the timing of the systemremaining locked thereto by, for example, positioning the sampling timeT across the waveform 10 through very small increments. Small incrementsof the sampling time T may be obtained by an appropriate voltageadjustment to the digital-toanalog converter 41 with respect to thedigital increments provided by the counter 40. The random short termfluctuations of the waveform 10 may thus efficaciously be reduced tozero by a conventional continuous integrator (not shown) having a longtime constant which integrator may be connected to the output of theamplifier 35.

Alternatively, numerous scans of the repetitive waveform 10 may beobtained and the samples thereof provided by the amplifier 35 beingstored, for example, in a digital computer. The computer may thereafterprovide the average value for each sampled point on the waveform, theaverage being taken with respect to the total number of scans obtained.in this manner, the random short term fluctuations of the waveform 10may effectively be reduced to zero.

The present invention embodied in the apparatus hereindescribed may findutility in a wide variety of applications. For example, it may bedesired to perform a computer analysis of a high frequency repetitivewaveform having an unfavorable signal to noise ratio. Compensatedsamples of the input waveform may be obtained during a slow scan thereofwhich samples may be provided by a continuous integrator connected tothe output of the amplifier 35 as discussed above. The compensatedsamples thus obtained may be applied thereafter as an input to a digitalcomputer via an analog-todigital converter for purposes of computeranalysis. Alternatively, numerous scans of the input waveform may beobtained and the samples thereof provided by the amplifier 35 applied asan input to the digital computer via an analog-to-digital converter. Thecomputer may obtain the point by point average of the scans therebyenhancing the signal to noise ratio of the input signal beforeperforming the computer analysis thereof.

As a further example of an application for the present invention, it isappreciated that the circuits thereof may be in corporated into theconventional sampling oscilloscope thereby providing an improvedinstrument for displaying high frequency wave fonns having low signal tonoise ratios whose wave shapes could heretofore only be approximatelydeter mined on the display thereof.

Although the present invention may be embodied by the apparatus as setforth hereinabove, other embodiments within the scope of the inventionmay be provided. For example, a stored program digital computer may beappropriately programmed to perform the method of the present invention.The computer may be adapted to insert binary numbers into first andsecond registers connected to the output thereof. The first register maybe caused by the computer to sequentially contain binary numbersrepresentative of the voltages 1,, t and 1 respectively and the secondregister may be caused by the computer to hold binary numbersrepresentative of the feedback signal. Two digital-to-analog convertersmay be coupled to the outputs of the two registers, respectively, theoutputs of which converters may in turn be combined by an operationalamplifier. The output of the operational amplifier may be connected tothe external sweep input of the time base unit 17 of the samplingoscilloscope 15. The sampling amplifier 20 may provide the sampledmagnitudes of the input waveform 10 to an input of the digital computervia an analog-to-digital converter. The computer may be appropriatelyprogrammed to perform the calculations described hereinabove withrespect to the method of the present invention.

While the invention has been described in its preferred embodiments, itis to be understood that the words which have been used are words ofdescription rather than limitation and that changes may be made withoutdeparting from the true scope and spirit of the invention in its broaderaspects.

l claim:

1. Apparatus for compensating amplitude and time drifts in sampledwaveform systems having a time base established with respect to saidwaveform, said apparatus comprising first, second and third samplingmeans responsive to said waveform for providing samples of the magnitudeof said waveform at first, second and third sampling times with respectto said time base, respectively,

first subtraction means responsive to said first and second samplingmeans for providing a first signal representative of the differencebetween said magnitudes sampled at said first and second sampling times,

reference means for storing an initail value of said first signal,

second subtraction means responsive to said first subtraction means andsaid reference means for providing an error signal representative of thedifference between said first signal and said stored initial valuethereof,

adjusting means responsive to said second subtraction means foradjusting said first, second and third sampling times, in accordancewith said error signal, and

third subtraction means responsive to said first and third samplingmeans for providing a signal representative of the difference betweensaid magnitudes sampled at said first and third adjusted sampling timesthereby providing a signal representative of a compensated sample ofsaid waveform.

2. Apparatus of the character recited in claim 1 further includingfirst, second and third positioning means for providing first, secondand third positioning signals respectively whose respective coactionswith said time base provide signals determinant respectively of saidfirst, second and third sampling times with respect to said time base.

3. Apparatus of the character recited in claim 2 further includingswitching means for sequentially coupling said first, second and thirdpositioning signals into coaction with said time base, and

triggering means synchronized with said switching means for sequentiallyproviding actuating trigger signals to said first, second and thirdsampling means respectively.

4. Apparatus of the character recited in claim 2 further including meansfor incrementing said third positioning signal thereby to provideadjacent samples of said waveform.

5. Apparatus of the character recited in claim 2 in which said adjustingmeans includes integrating means responsive to said error signal forintegrating said error signal thereby providing a feedback signal, and

combining means responsive to said feedback signal and said positioningsignals for combining said feedback signal with said positioning signalsthereby to reduce said error signal.

6. Apparatus of the character recited in claim 5 in which said adjustingmeans further includes inverting means for selectively inverting thepolarity of said feedback signal in accordance with the polarity of theslope of said waveform at said second sampling time.

7. A method for compensating amplitude and time drifts in sampledwaveform systems having a time base with respect to said waveform, saidmethod comprising the steps of sampling the magnitude of said waveformat a first sampling time with respect to said time base,

sampling the magnitude of said waveform at a second sampling time withrespect to said time base,

obtaining the difference between said magnitudes sampled at said firstand second sampling times respectively thereby providing a first signalrepresentative of the difference therebetween,

resampling the magnitudes of said waveform at said first and secondsampling times, respectively, obtaining the difference between saidmagnitude resampled at said first and second sampling times,respectively, thereby providing a second signal representative of thedifference therebetween,

obtaining the difference between said first and second signals therebyproviding an error signal representative of the difference therebetween,

generating a feedback signal in accordance with said error signal,sampling the magnitude of said waveform at a third sampling time withrespect to said time base, said third sampling time being adjusted inaccordance with said feedback signal, and

obtaining the difference between said magnitude sampled at said thirdsampling time and said magnitude resampled at said first sampling timethereby providing a first compensated sample of said waveform.

8. A method of the character recited in claim 7 further including thesteps of sampling the magnitudes of said waveform at adjusted first andsecond sampling times, respectively,

said first and second sampling times being adjusted in accordance withsaid feedback signal,

obtaining the difference between said magnitudes sampled at saidadjusted first and second sampling times respectively thereby providinga new second signal representative of the difference therebetween,

obtaining the difference between said first signal and said new secondsignal thereby providing a new error signal representative of thedifference therebetween,

combining said new error signal with said feedback signal,

sampling the magnitude of said waveform at a fourth sampling time withrespect to said time base, said fourth sampling time being adjusted inaccordance with said feedback signal, and

obtaining the difference between said magnitude sampled at said fourthsampling time and said magnitude sampled at said adjusted first samplingtime thereby providing a second compensated sample of said waveform.

9. A method of the character recited in claim 7 in which said step ofsampling the magnitude of said waveform at a first sampling timeincludes the step of selecting said first sampling time whereby saidwaveform is sampled on a portion thereof having a slope of smallmagnitude, and

said step of sampling the magnitude of said waveform at a secondsampling time includes the step of selecting said second sampling timewhereby said waveform is sampled on a portion thereof having a slope oflarge magnitude with respect to said slope of small magnitude.

10. A method of the character recited in claim 9 in which said step ofselecting said first sampling time comprises the step of selecting saidfirst sampling time whereby said waveform is sampled on a portionthereof having a slope of zero magnitude.

11. A method of the character recited in claim 8 in which said step ofsampling the magnitudes of said waveform at adjusted first and secondsampling times comprises the step of sampling the magnitudes of saidwaveform at adjusted first second sampling times respectively whereinsaid first and second sampling times are adjusted thereby to reduce saidnew error signal.

12. A method of the character recited in claim 7 in which said step ofgenerating a feedback signal comprises the step of integrating saiderror signal thereby providing said feedback signal.

1. Apparatus for compensating amplitude and time drifts in sampledwaveform systems having a time base established with respect to saidwaveform, said apparatus comprising first, second and third samplingmeans responsive to said waveform for providing samples of the magnitudeof said waveform at first, second and third sampling times with respectto said time base, respectively, first subtraction means responsive tosaid first and second sampling means for providing a first signalrepresentative of the difference between said magnitudes sampled at saidfirst and second sampling times, reference means for storing an initailvalue of said first signal, second subtraction means responsive to saidfirst subtraction means and said reference means for providing an errorsignal representative of the difference between said first signal andsaid stored initial value thereof, adjusting means responsive to saidsecond subtraction means for adjusting said first, second and thirdsampling times, in accordance with said error signal, and thirdsubtraction means responsive to said first and third sampling means forproviding a signal representative of the difference between saidmagnitudes sampled at said first and third adjusted sampling timesthereby providing a signal representative of a compensated sample ofsaid waveform.
 2. Apparatus of the character recited in claim 1 furtherincluding first, second and third positioning means for providing first,second and third positioning signals respectively whose respectivecoactions with said time base provide signals determinant respectivelyof said first, second and third sampling times with respect to said timebase.
 3. Apparatus of the character recited in claim 2 further includingswitching means for sequentially coupling said first, second and thirdpositioning signals into coaction with said time base, and triggeringmeans synchronized with said switching means for sequentially providingactuating trigger signals to said first, second and third sampling meansrespectively.
 4. Apparatus of the character recited in claim 2 furtherincluding means for incrementing said third positioning signal therebyto provide adjacent samples of said waveform.
 5. Apparatus of thecharacter recited in claim 2 in which said adjusting means includesintegrating means responsive to said error signal for integrating saiderror signal thereby providing a feedback signal, and combining meansresponsive to said feedback signal and said positioning signals forcombining said feedback signal with said positioning signals thereby toreduce said error signal.
 6. Apparatus of the character recited in claim5 in which said adjusting means further includes inverting means forselectively inverting the polarity of said feedback signal in accordancewith the polarity of the slope of said waveform at said second samplingtime.
 7. A method for compensating amplitude and time drifts in sampledwaveform systems having a time base with respect to said waveform, saidmethod comprising the steps of sampling the magnitude of said waveformat a first sampling time with respect to said time base, sampling themagnitude of said waveform at a second sampling time with respect tosaid time base, obtaining the difference between said magnitudes sampledat said first and second sampling times respectively thereby providing afirst signal representative of the difference therebetween, resamplingthe magnitudes of said waveform at said first and second sampling times,respectively, obtaining the difference between said magnitude resampledat said first and second sampling times, respectively, thereby providinga second signal representative of the difference therebetween, obtainingthe difference between said first and second signals thereby providingan error signal representative of the difference therebetween,generating a feedback signal in accordance with said error signal,sampling the magnitude of said waveform at a third sampling time withrespect to said time base, said third sampling time being adjusted inaccordance with said feedback signal, and obtaining the differencebetween said magnitude sampled at said third sampling time and saidmagnitude resampled at said first sampling time thereby providing afirst compensated sample of said waveform.
 8. A method of the characterrecited in claim 7 further including the steps of sampling themagnitudes of said waveform at adjusted first and second sampling times,respectively, said first and second sampling times being adjusted inaccordance with said feedback signal, obtaining the difference betweensaid magnitudes sampled at said adjusted first and second sampling timesrespectively thereby providing a new second signal representative of thedifference therebetween, obtaining the difference between said firstsignal and said new second signal thereby providing a new error signalrepresentative of the difference therebetween, combining said new errorsignal with said feedback signal, sampling the magnitude of saidwaveform at a fourth sampling time with respect to said time base, saidfourth sampling time being adjusted in accordance with said feedbacksignal, and obtaining the difference between said magnitude sampled atsaid fourth sampling time and said magnitude sampled at said adjustedfirst sampling time thereby providing a second compensated sample ofsaid waveform.
 9. A method of the character recited in claim 7 in whichsaid step of sampling the magnitude of said waveform at a first samplingtime includes the step of selecting said first sampling time wherebysaid waveform is sampled on a portion thereof having a slope of smallmagnitude, and said step of sampling the magnitude of said waveform at asecond sampling time includes the step of selecting said second samplingtime whereby said waveform is sampled on a portion thereof having aslope of large magnitude with respect to said slope of small magnitude.10. A method of the character recited in claim 9 in which said step ofselecting said first sampling time comprises the step of selecting saidfirst sampling time whereby said waveform is sampled on a portionthereof having a slope of zero magnitude.
 11. A method of the characterrecited in claim 8 in which said step of sampling the magnitudes of saidwaveform at adjusted first and second sampling times comprises the stepof sampling the magnitudes of said waveform at adjusted first secondsampling times respectively wherein said first and second sampling timesare adjusted thereby to reduce said new error signal.
 12. A method ofthe character recited in claim 7 in which said step of generating afeedback signal comprises the step of integrating said error signalthereby providing said feedback signal.